A television signal may include auxiliary information in addition to video program and audio program information. For example, an NTSC (National Television Standards Committee) television signal may include two bytes of closed captioning data during the latter half of each occurrence of line 21 of field 1. Closed caption data may be decoded and displayed to provide a visible text representation of a television program's audio content. Additional closed caption data and other types of similarly encoded auxiliary information, such as extended data services information (XDS), may be included in other line intervals such as line 21 of field 2. United States law requires caption decoders in all television receivers having displays larger than 13 inches and most television programming (including video tapes) now includes captioning data.
Although captioning was developed to aid the hearing impaired, captioning can also provide a benefit to non-hearing impaired viewers as well. Captioning for a multi-image display such as picture-in-picture (PIP) or picture-outside-picture (POP) displays is an example of this type of additional benefit. For example, activating a PIP feature produces an auxiliary image representing the video content of a secondary television program signal. The auxiliary image is a small picture that is inset into a portion of the main picture. However, only the audio program associated with the main picture is processed and coupled to the speakers of the television. The audio content of the secondary signal is lost. Because the audio program is important to the comprehension of a television program, the usefulness of a multi-image display feature such as a PIP display is severely limited by the lack of an associated audio program. An approach to solving this problem is to display captions, i.e., visible text, representing the PIP audio program in a portion of the display. However, the closed caption decoder in most television receivers processes only the caption information associated with the "main" picture, not the small picture signal.
An exception to this general rule can be found in certain television receivers manufactured by Sharp Corporation such as models 31H-X1200 and 35H-X1200. These Sharp television receivers display captions representing the audio of the PIP image by providing a switching capability that permits coupling the PIP signal to the main caption decoder. PIP captions are displayed full size (up to four rows of 32 large characters) at the top or bottom of the screen (a user selectable position). An example of PIP captioning produced by Sharp television receivers is shown in FIG. 1 which depicts a display including main image 100, PIP image 102 and PIP caption 104.